1. Field of the Invention
The present invention relates to a functional device package for various kinds of sensors, actuators, etc. manufactured by processing through etching an Si substrate (the Si substrate includes here and hereinafter an Si wafer, SiO2, or a wafer having an Si/SiO2/Si sandwich structure in which an SiO2 insulation layer is disposed on the inside) based on MEMS (micro electro mechanical systems) technology or the like.
2. Description of the Related Art
Functional devices such as various kinds of sensors, high frequency filters, and mirror functional devices have been manufactured by using the MEMS technology. For example, in an acceleration sensor, a structure including a weight is generally formed by using an etching technique, and acceleration that exerts on the weight can be read by reading an electrostatic capacitance change in an electrode connected to the weight or a resistance change of a strain detection device formed on a thin beam connected to the weight. In a high-frequency filter, for example, of a structure referred to as FBAR (film bulk acoustic resonator), a cavity is formed on an Si substrate, and an AlN piezoelectric film sandwiched between electrodes is formed so as to override the cavity. In a mirror device, a mirror portion of Si is driven, and an optical channel is adjusted to form images. Any of the MEMS functional devices includes movable portions such as a weight, a piezoelectric film, or a mirror. Operation characteristics of such movable portions are naturally influenced by the effect of an atmosphere surrounding them. Accordingly, it is necessary to keep an air pressure constant in a space where the movable portion is present, thus necessitating hermetic sealing.
Heretofore, to take out an electric signal from a functional device to the outside under the hermetic sealing, a package and a cover both made of ceramics have been used. In the ceramic package, electrode pads for connecting wire bonding are formed on the inside, and wiring connected to the electrode pads is taken out from the inside of the ceramic package to its outside so as not to cause leakage. On the outside of the ceramic package, electrode pads connected with the wiring led out of the inside are formed.
However, a demand for reducing the size and lowering the cost has been increasing in recent years, and packaging at a wafer level has become necessary as means for satisfying the demand. Packaging at the wafer level refers to a method of connecting another wafer for sealing to a wafer on which various functional devices are formed to complete packaging. Since functional devices as many as several thousands or several tens of thousands can be sealed collectively by the connection of the wafers, this is advantageous in terms of decreasing the cost. Further, a sealing portion can be designed precisely by photolithography, this is also advantageous in terms of the size reduction of the package.
As an example of means for connecting wafers, while several new methods such as Si—Si direct connection and anodic bonding between a glass wafer and Si have been presented, solder connection can be given, which is a proven connecting method conventionally used for packaging semiconductor parts.
As the techniques disclosed thus far, description is to be made on two methods, that is, a method of packaging a functional device by solder connection although not at a wafer level and a packaging method by soldering at a wafer level.
The following JP-A-10-303323 describes a package structure which is particularly suitable to a package for hermetically sealing a high frequency IC that forms an oscillation circuit and which can maintain required hermetic sealing with a sufficient adhesion strength even when the thickness of the cap is increased as a vibration countermeasure to increase the connection area with a header. As the method, double circular recessed grooves are formed in the inner circumference and the outer circumference of the connection surface of the header to be connected to the cap, the header for the inner connection portion surrounded with the circular recessed grooves in the inner and the outer circumferences and the cap are welded by a brazing material, and the header for the outer connection portion more external than the circular recessed groove in the outer circumference and the cap are welded by laser welding. Further, it is shown that the circular recessed grooves in the inner and the outer circumferences formed on the bonding portion are grooves for preventing the flow of the brazing material to the high frequency IC portion.
Further, JP-A-2004-235440, with a view to providing a micropackage in which wafers of a large diameter are favorably connected, discloses a method of hermetically sealing a device wafer in which a plurality of active portions of a microsensor or a microactuator are formed on the wafer and a cover member at the periphery of each active portion of the device wafer by a connection portion. It is disclosed that solder is used for the connection.